Just Released: The importance of Atrazine Paper – M. Owen Prof. of Agronomy – Iowa State University

Herbicide-resistant weeds: An evolving problem of importance in Iowa crop production

Micheal D. K. Owen, professor and extension weed specialist, Agronomy, Iowa State University


The changes in Iowa agriculture over the last three decades have been monumental and the implications of these changes often overlooked during the course of developing the plans for next year. Consider that in the 1970’s, aggressive tillage predominated the production systems in Iowa, conservation tillage was an interesting but not generally practiced idea and herbicides had to be mechanically incorporated into the soil. In the 1980’s, the acetolactate synthase (ALS) inhibiting herbicide families were introduced. The imidazolinone and sulfonyl urea herbicide families were applied to an estimated 90% of the soybean acres and more than 65% of the corn acres. In many instances, these herbicides were applied repeatedly on fields during the year and certainly recurrently from year to year. Despite warnings that this production practice would result in significant problems (e.g. evolved ALS-resistant weed biotypes), commercial agriculture continued with the unsustainable practice of using one type of herbicide exclusively and the inevitable resistant weed problem evolved as predicted. By the time glyphosate-resistant (GR) crops were introduced, ALS resistance was widespread and much of the utility of these important products had been lost. However, the GR crop technologies and concomitant use of glyphosate became available and adoption in global agriculture was unprecedented. Importantly, the trends toward conservation tillage practices were strongly supported by the "new" system.

Usage of glyphosate rose to the point that there were no other herbicides used on more than 10% of the soybean acres and only atrazine continued to demonstrate a strong presence in corn (Young, 2006). Once again, naysayers suggested that because the GR-based crop production systems were essentially devoid of diversity for weed management, glyphosate-resistant weeds would evolve (Owen, 1997). These warnings were again unheeded and the inevitable resistant weeds did indeed evolve to the extent that the GR technologies are threatened. Unfortunately, this time, given the unprecedented adoption of GR-based crop systems and glyphosate utilization, the industry had essentially withdrawn from herbicide discovery and development such that no new answers would come forward. Given the dire straits that currently exist in weed management, now is the time to objectively review the sustainability of the system and determine if perhaps it is time to change perspectives on a more diverse management plan for weeds.

What are the options?

In Iowa corn and soybean production, there are a number of effective, but sparingly utilized, tools and tactics available to manage the ever increasing herbicide resistant weed problem. This is unlike the situation in the Mississippi Delta and Southeastern states where cotton production is threatened (Culpepper and York, 2007). The rhetorical question as to why the plethora of tools and tactics available to Iowa agriculture have not been used, even when it was correctly suggested that the evolution of GR weeds was inevitable, is a function of demographic changes in agriculture (i.e. size of farms and time availability) as well as a desire for the convenience and simplicity that the GR crop-based systems provided (Owen et al., 2009; Owen et al., 2010). Growers continued to maintain a position of denial that these ever increasing problems would ever impact their farms; it should now be clear that the problems are here and changes must occur now or potentially Iowa agriculture will experience the same severe consequences that growers in Georgia are now facing.

Herbicide options

There are numerous "alternate" herbicides that can help resolve the glyphosate resistance in weeds. "Alternate" is another way of saying "old" and these established herbicides are indeed useful if properly included in a longer-term weed management plan. The list of herbicides currently registered for corn and soybean is long and represented by a number of herbicide families and mechanisms of action (MOA). Generally, the more herbicide diversity that is included in a long term weed management plan, the better. However, many of these herbicides have already been improperly used and thus have selected for resistant weed biotypes. Thus, the simple inclusion of other herbicides will not necessarily resolve weed management problems. A partial list of available "alternate" herbicides is presented in Table 1.